The present invention relates to a novel material for electrode, particularly a material for electrode suitable for a gas diffusion layer of fuel battery.
An another look has been given to batteries as clean energy sources, and batteries based on various theories have been proposed and developed. For example, fuel batteries giving electric energy by a reaction of hydrogen or hydrocarbon gas as a fuel gas with oxygen are expected as energy sources together with solar batteries.
The fuel batteries basically have a structure having a pair of gas diffusion electrodes comprising a reaction layer (catalyst layer) and gas diffusion layer (electrode layer) with various electrolytes being filled between the reaction layers. A collecting plate is usually disposed at the outside of the gas diffusion layer.
From the point that hydrogen (hydrocarbon) gas is reacted with oxygen gas (air) to give water (steam), the gas diffusion layer requires at least the following characteristics.
(1) Having Gas Permeability
It is necessary for the gas diffusion layer to have a porous structure to give a proper gas diffusion speed.
(2) Having Hydrophobic Property
It is necessary that the gas diffusion layer is hydrophobic so that generated steam is not condensed in the gas diffusion layer and gas diffusion (permeation) is not hindered. In order to make the gas diffusion layer hydrophobic, polytetrafluoroethylene is usually blended.
(3) Having Electric Conductivity
Good electric conductivity is required to take out electric energy. To give electric conductivity, a carbon fiber, carbon whisker or carbon black is usually blended.
Further in order to enhance an efficiency of conversion to electric energy and an energy taking-out efficiency, not only a material but also a shape of the gas diffusion layer is important.
(4) Processability into Various Shapes and Easy Processing are Necessary
As the fuel batteries, there are an alkali type, phosphoric acid type, fused carbonate type, solid polyelectrolyte type, and the like depending on kind of electrolyte. Among them, attention has been drawn to the solid polyelectrolyte type from the points that it has an operation temperature around at 70xc2x0 to 100xc2x0 C., a high current density can be obtained, its service life is long, it is free from deterioration due to starting and stopping, starting at low temperature is easy, operation at low load can be made, a precise pressure differential control is not necessary, inexpensive material can be used, it is subject to less corrosion, and there is no poisoning by CO2.
As the gas diffusion layer (electrode) of such a solid polyelectrolyte type fuel battery, there have been proposed one which is produced by kneading a graphite whisker and polytetrafluoroethylene (PTFE) powder with a shearing force being applied thereto to fibrillate the PTFE powder and then hot-pressing a sheet in which the graphite whisker is entangled with the fibrillated PTFE to a sheet for the reaction layer (JP-A-7-23081 1), one which is produced by hot-pressing a sheet-like substrate for the reaction layer to a sheet-like substrate for the gas diffusion layer obtained by mixing a carbon fiber with water-repellent carbon black and PTFE powder by using a porous metal or one obtained by hot-pressing a water-repellent carbon black and PTFE powder to a porous carbon substrate such as a carbon paper (JP-A-7-220734), one which is produced by using a substrate of porous carbon sheet (carbon paper) comprising only a carbonaceous material obtained by impregnating a mass of carbon fibers with a phenolic resin and then carbonating the phenolic resin by heating (JP-A-9-157052), and the like.
However according to the method disclosed in JP-A-7-230811, the PTFE powder is fibrillated but cannot be said to be a so-called fiber, and sufficient entangling with the graphite whisker cannot be obtained. Therefore PTFE must be melted by hot-pressing to be integrated. As described also in JP-A-7-220734, since the hot-pressing destroys the porous structure and gas diffusion passages are broken, gas diffusion ability is lowered.
In order to improve that point, in JP-A-7-220734, hot-pressing is carried out with a porous plate. However a porous structure is not sufficient and also not satisfactory from the viewpoint of strength.
The carbon paper described in JP-A-9-157052 has good porosity but due to carbonization, is wholly fragile and poor in processability.
An object of the present invention is to provide the material for electrode which is excellent in gas permeability (porosity), hydrophobic property, electric conductivity, chemical resistance and further in mechanical strength (flexibility), moldability and processability.
The present invention relates to the material for electrode comprising a fluorine-containing resin fiber and an electrically conductive fiber, further the material for electrode produced from a non-woven fabric comprising a fluorine-containing resin fiber and an electrically conductive fiber.
In the present invention, the fluorine-containing resin xe2x80x9cfiberxe2x80x9d does not encompass such a fibrillated PTFE as described in the above-mentioned JP-A-7-230811 which is obtained by applying a shearing force to a PTFE powder to fibrillate it. The fibrillated PTFE has an ability to agglomerate carbon powder particles, whiskers and fibers and further platinum catalyst powder with each other, but cannot give a mechanical strength to the agglomerate. Therefore a molded article of fibrillated PTFE requires a separate substrate for supporting thereof.
In the present invention, the xe2x80x9cfluorine-containing resin fiberxe2x80x9d can give a sufficient mechanical strength such as tensile strength, elongation strength and bending strength to a molded polymer. Particularly the fiber having a branched structure is preferable for enhancing intermingling property between the fibers to form a molded article.